2,958 research outputs found
Microlensing planet detection via geosynchronous and low Earth orbit satellites
Planet detection through microlensing is usually limited by a well-known
degeneracy in the Einstein timescale , which prevents mass and distance of
the lens to be univocally determined. It has been shown that a satellite in
geosynchronous orbit could provide masses and distances for most standard
planetary events ( days) via a microlens parallax measurement.
This paper extends the analysis to shorter Einstein timescales,
day, when dealing with the case of Jupiter-mass lenses. We then study the
capabilities of a low Earth orbit satellite on even shorter timescales, days. A Fisher matrix analysis is employed to predict how the
1- error on parallax depends on and the peak magnification of the
microlensing event. It is shown that a geosynchronous satellite could detect
parallaxes for Jupiter-mass free floaters and discover planetary systems around
very low-mass brown dwarfs. Moreover, a low Earth orbit satellite could lead to
the discovery of Earth-mass free-floating planets. Limitations to these results
can be the strong requirements on the photometry, the effects of blending, and
in the case of the low orbit, the Earth's umbra.Comment: 5 pages, 3 figures. Minor language edits. Accepted for publication in
Astronomy & Astrophysic
Tracking the phase-transition energy in disassembly of hot nuclei
In efforts to determine phase transitions in the disintegration of highly
excited heavy nuclei, a popular practice is to parametrise the yields of
isotopes as a function of temperature in the form
, where 's are the measured yields
and and are fitted to the yields. Here would be
interpreted as the phase transition temperature. For finite systems such as
those obtained in nuclear collisions, this parametrisation is only approximate
and hence allows for extraction of in more than one way. In this work we
look in detail at how values of differ, depending on methods of
extraction. It should be mentioned that for finite systems, this approximate
parametrisation works not only at the critical point, but also for first order
phase transitions (at least in some models). Thus the approximate fit is no
guarantee that one is seeing a critical phenomenon. A different but more
conventional search for the nuclear phase transition would look for a maximum
in the specific heat as a function of temperature . In this case is
interpreted as the phase transition temperature. Ideally and would
coincide. We invesigate this possibility, both in theory and from the ISiS
data, performing both canonical () and microcanonical ()
calculations. Although more than one value of can be extracted from the
approximate parmetrisation, the work here points to the best value from among
the choices. Several interesting results, seen in theoretical calculations, are
borne out in experiment.Comment: Revtex, 10 pages including 8 figures and 2 table
Cepheid models based on self-consistent stellar evolution and pulsation calculations: the right answer?
We have computed stellar evolutionary models for stars in a mass range
characteristic of Cepheid variables (3) for different
metallicities representative of the Galaxy and the Magellanic Clouds
populations. The stellar evolution calculations are coupled to a linear non
adiabatic stability analysis to get self-consistent mass-period-luminosity
relations. The period - luminosity relation as a function of metallicity is
analysed and compared to the recent EROS observations in the Magellanic Clouds.
The models reproduce the observed width of the instability strips for the SMC
and LMC. We determine a statistical P-L relationship, taking into account the
evolutionary timescales and a mass distribution given by a Salpeter mass
function. Excellent agreement is found with the SMC PL relationship determined
by Sasselov et al. (1997). The models reproduce the change of slope in the P-L
relationship near days discovered recently by the EROS
collaboration (Bauer 1997; Bauer et al. 1998) and thus explain this feature in
term of stellar evolution. Some discrepancy, however, remains for the LMC
Cepheids. The models are also in good agreement with Beat Cepheids observed by
the MACHO and EROS collaborations. We show that most of the 1H/2H Beat Cepheids
have not yet ignited central helium burning; they are just evolving off the
Main Sequence toward the red giant branch.Comment: 18 pages, Latex file, uses aasms4.sty, accepted for publication in
ApJ Letter
Decline in the strength of genetic controls on aspen environmental responses from seasonal to centuryâlong phenomena
Understanding intra-specific variation in climate sensitivity could improve the prediction of tree responses to climate change. We attempted to identify the degree of genetic control of tree phenology and growth of trembling aspen (Populus tremuloides Mchx.) in a natural stand of this species in northwestern Quebec. We mapped and genotyped 556 aspen trees growing within the plot, using seven nuclear microsatellite loci for clone identification. We selected 13 clones (n of trees per clone >5, in total 350 trees) and evaluated the explanatory power of clone identity in (a) variability of spring leaf phenology and (b) short- and long-term growth responses. The clone's identity explained 43% of the variability in spring leaf phenology, between 18% and 20% of variability in response to monthly climate variables significantly affecting growth, between 8% and 26% of growth response to insect outbreaks, and 12% in the long-term growth rates. Strong clonal control of aspen phenology and moderate control of growth responses to monthly weather do not result in an equally large impact on long-term growth rates. The result suggests an important role of environmental extremes and within community interactions as factors averaging aspen growth performance at the stand level
Classical Cepheid Pulsation Models: IX. New Input Physics
We constructed several sequences of classical Cepheid envelope models at
solar chemical composition () to investigate the dependence of
the pulsation properties predicted by linear and nonlinear hydrodynamical
models on input physics. To study the dependence on the equation of state (EOS)
we performed several numerical experiments by using the simplified analytical
EOS originally developed by Stellingwerf and the recent analytical EOS
developed by Irwin. Current findings suggest that the pulsation amplitudes as
well as the topology of the instability strip marginally depend on the adopted
EOS.
We also investigated the dependence of observables predicted by theoretical
models on the mass-luminosity (ML) relation and on the spatial resolution
across the Hydrogen and the Helium partial ionization regions. We found that
nonlinear models are marginally affected by these physical and numerical
assumptions. In particular, the difference between new and old models in the
location as well as in the temperature width of the instability strip is on
average smaller than 200 K. However, the spatial resolution somehow affects the
pulsation properties. The new fine models predict a period at the center of the
Hertzsprung Progression (9.84 days) that reasonably agree with
empirical data based on light curves ( days;
\citealt{mbm92}) and on radial velocity curves ( days;
\citealt{mall00}), and improve previous predictions by Bono, Castellani, and
Marconi (2000, hereinafter BCM00).Comment: 35 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
Spinodal decomposition of expanding nuclear matter and multifragmentation
Density fluctuations of expanding nuclear matter are studied within a
mean-field model in which fluctuations are generated by an external stochastic
field. Fluctuations develop about a mean one-body phase-space density
corresponding to a hydrodinamic motion that describes a slow expansion of the
system. A fluctuation-dissipation relation suitable for a uniformly expanding
medium is obtained and used to constrain the strength of the stochastic field.
The distribution of the liquid domains in the spinodal decomposition is
derived. Comparison of the related distribution of the fragment size with
experimental data on the nuclear multifragmentation is quite satisfactory.Comment: 19 RevTex4 pages, 6 eps figures, to appear in Phys. Rev.
HI Detection in two Dwarf S0 Galaxies in Nearby Groups: ESO384-016 and NGC 59
An \hi survey of 10 dE/dS0 galaxies in the nearby Sculptor and Centaurus A
groups was made using the Australia Telescope Compact Array (ATCA). The
observed galaxies have accurate distances derived by Jerjen et al (1998; 2000b)
using the surface brightness fluctuation technique. Their absolute magnitudes
are in the range . Only two of the ten galaxies were
detected at our detection limit ( \msol for the Centaurus
group and \msol for the Sculptor group), the two dS0
galaxies ESO384-016 in the Centaurus A Group and NGC 59 in the Sculptor Group,
with \hi masses of \msol and \msol respectively. Those two detections were confirmed using the Green
Bank Telescope. These small \hi reservoirs could fuel future generations of low
level star formation and could explain the bluer colors seen at the center of
the detected galaxies. Similarly to what is seen with the Virgo dEs, the two
objects with \hi appear to be on the outskirt of the groups.Comment: 25 pages (11 figures), accepted by A
Benchmark of the PenRed Monte Carlo framework for HDR brachytherapy
[EN] Purpose: The purpose of this study is to validate the PenRed Monte Carlo framework for clinical applications in brachytherapy. PenRed is a C++ version of Penelope Monte Carlo code with additional tallies and utilities.
Methods and materials: Six benchmarking scenarios are explored to validate the use of PenRed and its improved bachytherapy-oriented capabilities for HDR brachytherapy. A new tally allowing the evaluation of collisional kerma for any material using the track length kerma estimator and the possibility to obtain the seed positions, weights and directions processing directly the DICOM file are now implemented in the PenRed distribution. The four non-clinical test cases developed by the Joint AAPM-ESTRO-ABG-ABS WG-DCAB were evaluated by comparing local and global absorbed dose differences with respect to established reference datasets. A prostate and a palliative lung cases, were also studied. For them, absorbed dose ratios, global absorbed dose differences, and cumulative dose-volume histograms were obtained and discussed.
Results: The air-kerma strength and the dose rate constant corresponding to the two sources agree with the reference datatests within 0.3% (Sk) and 0.1% (¿). With respect to the first three WG-DCAB test cases, more than 99.8% of the voxels present local (global) differences within ±1%(±0.1%) of the reference datasets. For test Case 4 reference dataset, more than 94.9%(97.5%) of voxels show an agreement within ±1%(±0.1%), better than similar benchmarking calculations in the literature. The track length kerma estimator scorer implemented increases the numerical efficiency of brachytherapy calculations two orders of magnitude, while the specific brachytherapy source allows the user to avoid the use of error-prone intermediate steps to translate the DICOM information into the simulation. In both clinical cases, only minor absorbed dose differences arise in the low-dose isodoses. 99.8% and 100% of the voxels have a global absorbed dose difference ratio within ±0.2% for the prostate and lung cases, respectively. The role played by the different segmentation and composition material in the bone structures was discussed, obtaining negligible absorbed dose differences. Dose-volume histograms were in agreement with the reference data.
Conclusions: PenRed incorporates new tallies and utilities and has been validated for its use for detailed and precise high-dose-rate brachytherapy simulations.This work is partially supported by the Ministerio de
Ciencia e InnovaciĂłn of Spain (MCIN) grants PID2020-
113126RB-I00 and PID2021-125096NB-I00 funded by
MCIN/AEI/10.13039/501100011033. V. G. acknowledges
partial support from AEI-MICINN under grant PID2020-1
13334GB-I00/AEI/10.13039/501100011033 and by Generalitat Valenciana through the project PROMETEO/2019/087. The work has also been supported by the
Natural Sciences and Engineering Research Council of
Canada (NSERC) grant RGPIN-2019-05038. Francisco
Berumen acknowledges support by the Fonds de Recherche
du Québec ¿ Nature et Technologies (FRQNT). J.V. and
F.B. would like to acknowledge funding by MCIN/
AEI/10.13039 and the Generalitat Valenciana (GVA)
grant PROMETEO/2021/064. Our colleague Prof. Vicent
Giménez Gómez passed away on November 16, 2022,
during the proofs of this study. We dedicate it to his
memory. The Universitat de ValĂšncia lost a dedicated and
accomplished physicist.Oliver-Gil, S.; GimĂ©nez-Alventosa, V.; Berumen, F.; GimĂ©nez, V.; Beaulieu, L.; Ballester, F.; Vijande, J. (2023). Benchmark of the PenRed Monte Carlo framework for HDR brachytherapy. Zeitschrift fĂŒr Medizinische Physik. 33(4):511-528. https://doi.org/10.1016/j.zemedi.2022.11.00251152833
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